1. Field of the Invention
This invention relates to a radiation image storage panel for use in radiation image recording and reproducing techniques, in which stimulable phosphors are utilized.
2. Description of the Related Art
In lieu of conventional radiography, radiation image recording and reproducing techniques utilizing stimulable phosphors have heretofore been used in practice. The radiation image recording and reproducing techniques are described in, for example, U.S. Pat. No. 4,239,968. The radiation image recording and reproducing techniques utilize a radiation image storage panel (referred to also as the stimulable phosphor sheet) provided with a stimulable phosphor. With the radiation image recording and reproducing techniques, the stimulable phosphor of the radiation image storage panel is caused to absorb radiation, which carries image information of an object or which has been radiated out from a sample, and thereafter the stimulable phosphor is exposed to an electromagnetic wave (stimulating rays), such as visible light or infrared rays, which causes the stimulable phosphor to produce the fluorescence (i.e., to emit light) in proportion to the amount of energy stored thereon during its exposure to the radiation. The produced fluorescence (i.e., the emitted light) is photoelectrically detected to obtain an electric signal. The electric signal is then processed, and the processed electric signal is utilized for reproducing a visible image of the object or the sample. The radiation image storage panel, from which the electric signal has been obtained, is subjected to an erasing operation for erasing energy remaining on the radiation image storage panel, and the erased radiation image storage panel is utilized again for the image recording. Specifically, the radiation image storage panel is used repeatedly.
The radiation image recording and reproducing techniques have the advantages in that a radiation image containing a large amount of information is capable of being obtained with a markedly lower dose of radiation than in the conventional radiography utilizing a radiation film and an intensifying screen. Also, with the conventional radiography, the radiation film is capable of being used only for one recording operation. However, with the radiation image recording and reproducing techniques, the radiation image storage panel is used repeatedly. Therefore, the radiation image recording and reproducing techniques are advantageous also from the view point of resource protection and economic efficiency.
As described above, the radiation image recording and reproducing techniques are advantageous techniques for forming images. As in the cases of the intensifying screens utilized in the conventional radiography, it is desired that the radiation image storage panels utilized in the radiation image recording and reproducing techniques have the performance, such that the radiation image storage panels have a high sensitivity, yield good image quality, and endure a long period of use without the image quality of the radiation images becoming bad.
However, the stimulable phosphors utilized for the production of the radiation image storage panels ordinarily have high levels of water vapor absorbing characteristics and absorb moisture contained in air when being left within a room under ordinary weather conditions. Therefore, the stimulable phosphors have the problems in that the sensitivity of the stimulable phosphors with respect to the radiation becomes low as the amount of moisture absorbed by the stimulable phosphors becomes large, and the characteristics of the stimulable phosphors deteriorate markedly with the passage of time.
Also, ordinarily, latent images of the radiation images having been recorded on the stimulable phosphors have the properties such that the latent images fade with the passage of time after the stimulable phosphors have been exposed to the radiation. Therefore, as the time occurring between when the stimulable phosphors are exposed to the radiation and when the stimulable phosphors are exposed to the stimulating rays becomes long, the intensities of the radiation image signals detected from the stimulable phosphors become low. In cases where the stimulable phosphors absorb water vapor, the rate of the fading of the latent images having been recorded on the stimulable phosphors becomes high. Therefore, in cases where the radiation image storage panels, whose stimulable phosphors have absorbed water vapor, are used, there has arisen a tendency toward low reproducibility of the image signals at the time of the readout of the radiation images.
In order for the deterioration phenomenon of the stimulable phosphors due to water vapor absorption to be eliminated, there have heretofore been proposed techniques, wherein a stimulable phosphor layer is sealed with a plastic protective film. The techniques, where in a stimulable phosphor layer is sealed with a plastic protective film, are proposed in, for example, Japanese Patent Nos. 2843998, 2886165, and 2829607. The techniques for sealing with the plastic protective film have the advantages in that, for example, the plastic protective film is lighter in weight than a glass protective film and absorbs less of X-rays than the glass protective film. However, the techniques for sealing with the plastic protective film have the problems in that the plastic protective film exhibits a water vapor transmission rate higher than the water vapor transmission rate of the glass protective film, and deterioration of the stimulable phosphor is apt to occur more quickly than with the technique for sealing with the glass protective film. Also, in cases where a casting polypropylene (CPP), or the like, is subjected to heat fusion bonding, and a plastic protective film is thereby formed, since the CPP is thick, the thickness of the protective film as a whole is apt to become large, and the problems occur in that the light emitted by the stimulable phosphor spreads, and the obtained image becomes unsharp. Further, since the radiation image storage panel is used repeatedly as described above, from the viewpoint of prevention of image deterioration, it is necessary for the problems to be prevented from occurring in that the surface of the protective layer is scratched due to contact with a mechanical part, such as a conveying roller.
Also, in order for the deterioration phenomenon of the stimulable phosphors due to water vapor absorption to be eliminated, for example, there has heretofore been employed a technique, wherein a stimulable phosphor is covered with a film of a polytrifluorochloroethylene, or the like, acting as a water vapor proof protective layer having a low water vapor transmission rate, and the amount of moisture reaching the stimulable phosphor layer is thus reduced. However, the aforesaid technique for eliminating the deterioration phenomenon of the stimulable phosphors due to moisture absorption has the problems in that the cost of the aforesaid film of the polytrifluorochloroethylene, or the like, is high, and the thickness of the film is large. The aforesaid technique for eliminating the deterioration phenomenon of the stimulable phosphors due to moisture absorption also has the problems in that the film of the polytrifluorochloroethylene, or the like, is produced by use of Freon as a raw material, and therefore causes environmental pollution to occur.
Further, a constitution comprising two kinds of protective layers having different levels of water vapor absorbing characteristics, wherein one protective layer having a higher level of water vapor absorbing characteristics than the water vapor absorbing characteristics of the other protective layer is located on the side of a phosphor layer, is described in, for example, Japanese Patent Publication No. 4(1992)-76440. Furthermore, a constitution, wherein a protective layer contains a silicon compound containing nitrogen and oxygen, is described in, for example, Japanese Patent No. 1927597. However, water vapor proof characteristics, which are achieved by each of the constitutions described above, are not necessarily of a satisfactory level. Also, a technique for utilizing a laminated film for an electric field fluorescent lamp, wherein the laminated film is formed by laminating two to eight films, each of which has been prepared by forming a thin layer of a metal oxide, silicon nitride, or the like, on a polyethylene terephthalate (PET) film with vacuum evaporation, is described in, for example, Japanese Unexamined Patent Publication No. 10(1998)-12376. However, with the laminated film described above, the problems with regard to image defects due to the water vapor proof protective film, image defects due to a condition of adhesion between the water vapor proof protective film and a phosphor surface, and the like, occur. Therefore, the laminated film described above cannot be employed as a water vapor proof protective film for the radiation image storage panels, which are exclusively used for obtaining medical images for making a diagnosis of an illness.
Further, as a constitution for used in a radiation image storage panel, a constitution, wherein a laminated film comprising a plurality of resin films, which contain at least one metal oxide evaporated resin film and have been adhered to one another in a layer form, is located on the side of a phosphor layer surface, is proposed in, for example, Japanese Unexamined Patent Publication No. 11(1999)-3446598. However, with the proposed constitution, wherein the laminated film is adhered by an adhesive layer to the phosphor layer surface, the problems occur in that nonuniformity occurs with images, depending upon the condition of the adhesion of the laminated film. Also, with the proposed constitution, the problems occur in that the thickness of the entire water vapor proof layer becomes large, and the image quality becomes bad.